Project Summary/Abstract Advanced prostate cancer represents a major cause of cancer death in men, usually as a consequence of treatment failure, which gives rise to aggressive disease variants that are highly metastatic. Among the major biological processes that are causally dysregulated in advanced prostate cancer are those related to DNA repair. Notably, BRCA1 and BRCA2, are among the most frequently altered DNA repair genes in advanced prostate cancer, and are also key targets for its treatment. To study their functions in prostate cancer, we have generated genetically-engineered mouse models (GEMMs) based on inducible loss-of-function of Brca1 and Brca2 in the prostate. In preliminary studies, we have found that loss-of-function of Brca2 results in aggressive prostate tumors with highly penetrant metastases, including to bone, that are coincident with increased DNA damage and accelerated by androgen deprivation. In parallel, we have developed computational tools to elucidate the molecular determinants of Brca1 and Brca2 functions in prostate cancer. We have also developed a new precision oncology platform called OncoLoop, to computationally match individual patients to individual GEMMs, and to predict and validate drugs that target specific patient-GEMM pairs. Leveraging these models and resources, we will systematically investigate the functions of BRCA1 and BRCA2 in prostate cancer, guided by the hypothesis that defective DNA repair plays an important role in prostate cancer progression and treatment, particularly in the context of androgen deprivation. In Aim 1, we will investigate the consequences of loss-of-function of Brca1 and Brca2 in GEMMs, GEMM-derived organoid models, and patient-derived human organoid models for prostate tumorigenesis, metastasis, and DNA damage response. These studies will provide fundamental insights into the functions of BRCA1 and BRCA2 in prostate cancer and elucidate the relationship of DNA repair mechanisms for disease progr